This invention relates to a process for preparing a high voltage ignition cable (hereinafter referred to as an "ignition cable") which is used to suppress radio interference generated by electrical ignition in an internal combustion engine, e.g., in a car, etc.
When conductive substances such as salts (e.g., for the prevention of freezing of roads in a cold district), sludge, etc., attach onto the external surface of a jacket of the ignition cable and the impedance thereof relative to the ground potential is lowered, the charged current flows out thereto according to the electrostatic capacity between a resistive conductor core (hereinafter referred to as a "core," for simplicity) and the external surface of the jacket.
Therefore, as the electrostatic capacity increases, a reduction in the ignition voltage increases, resulting in poor ignition. In order to eliminate such poor ignition, it is necessary to use an ignition cable having as low electrostatic capacity as 80 pF/m or less.
One way of lowering the electrostatic capacity is to increase the outer diameter of the ignition cable. However, increasing the outer diameter is not desirable, since the outer diameter of the ignition cable is usually about 7 or 8 mm, and the ignition cable obtained cannot be exchanged with conventional ones, and requires additional space.
One method of lowering the electrostatic capacity while maintaining the outer diameter of the ignition cable at a predetermined level is to reduce the outer diameter of the core. However, various problems arise when merely reducing the outer diameter of the core of the conventional arts.
Glass fiber bundles have heretofore been used conventionally as a tension member constituting the core. When the diameter of the core prepared using the glass fiber bundle is reduced to lower the electrostatic capacity of the ignition cable, the core may be cut in the course of extrusion or vulcanization of the insulator layer, jacket, or the like. This makes the commercial production of such an ignition cable difficult.
The above defect encountered in the use of the glass fiber bundle can be overcome by using an aromatic polyamide fiber bundle of high strength as a tension member of the core, and an ignition cable having a low electrostatic capacity of about 80 pF/m can be obtained.
It has been found, however, that the thus-obtained ignition cable of a low electrostatic capacity suffers from the disadvantage that its high voltage-withstanding ability is unstable, and it is insufficiently durable for long and repeated use.